These hydrostatic machines can be used both as a hydraulic pump and, preferably, as a hydraulic motor and are particularly popular as low-speed "torque motors". Liquids and gases are used as the operating fluid. The particular advantage is a relatively large intake volume per revolution and hence a relatively high drive torque. These hydrostatic machines have the advantage that the shaft to the left and right of the displacement part and of the control part can be mounted in roller bearings having large dimensions, so that not only is there exact shaft mounting for the hydraulic part but a large bearing spacing, which permits high radial forces at the driven and output ends of the shaft, due to the considerable lever action of the shaft, is achieved. Not only is it possible to permit considerable belt and drive hub for hydrostatic wheel drives.
A known machine of this type (German Offenlegungsschrift 1,703,573) has a so-called rotor tooth system between the stationary housing and the outer tooth system of the rotary piston. This tooth system operates there as a displacement part. The rotary piston also has a rotor tooth system in its inner region, its inner rotor being connected nonrotatably, as a single piece, to the driven or output shaft. In this machine, an attempt is made to ensure that supply to, and removal from, the tooth system of the displacement part takes place via control slots which are arranged on the rotary piston itself. For reasons relating to design and gear kinematics the eccentricities of both rotor tooth systems must be identical. Thus, the tooth height of the tooth system of the displacement part depends on the tooth height of the very much smaller tooth system on the shaft, so that the delivery area, i.e. the specific volume per revolution of the tooth system of the displacement part, is still relatively small. Furthermore, the achievable flow cross-sections are disadvantageous owing to the commutator control envisaged there, so that there are high throttle losses.